What describes using organisms that were only alive during a specific time period to estimate the unknown age of items?

The correct answer is index fossils. A fossil index is a paleontological or archaeological remain whose presence can be used to date with some precision the stratigraphic unit in which it is found because they are particular or exclusive of a certain period of geological history, or of Prehistory, or indicators of a certain paleoenvironment.

In general, the taxa or cultural modes of the past that gave rise to the guide fossils had a very short duration on a geological -or historical- scale, which means that their presence is limited to chronological -or cultural- coordinates, in the case of prehistoric cultures-, very limited. That is, if a guide fossil appears in a stratum, that stratum can be dated with enough accuracy.

the arab empire adopted the muslim religion as their main belief. arabs would influence the people from the territories they invaded towards islam (the muslim belief).

nevertheless, throughout the history of the arab empire, rulers brought intellectual individuals from other religions to their centers of learning. these other foreigners included jews, christians, greeks, persians, and indians.   these resulted in the adoption of important knowledge developed by these cultures by the arabic empire. for example, the works of aristotle were translated from greek to arab and indian scientists brought the numerical concept of "zero" to the arabs.

nowadays, 93% of the arab population is muslim, which at the same time is subdivided in sects including the sunni, zaini, alawi, shia. the remaining 7% is composed mainly of christians.